Alignment and Design of Professional Sound Systems
I found a mistake in the MAPP plot in the intro gallery: one pic had the wrong autocad background and showed the lines crossing in phase where the acoustical was cancelling. I am sorry for any confusion this may have caused. THIS IS THE RIGHT ONE (aslo found in the ols post0
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Here is a quick preview of my next post. These plots contain concentric circles. Each represents a wavelength traveled from the speaker at 125 Hz (2.73 meters). The first one shows 7 speakers spaced at 1/2 wavelength (1.37m) and all set to 0 ms delay. The concentric are all the same for the 7 speakers and we can see where the beam will go – the top center, where all the speakers are in phase. The MAPP plot then shows you the result. We can also see where the nulls will be – the areas where the wavelength rings are spread all over.
The second set is the same speakers, same spacing but with a delay taper of .67 ms per box. So it’s 0.0, 0.67,1.33 …..up to 4.00 ms. At this freq this translates to 0, 30, 60, 90….. up to 180 degrees (30 deg per box). In this case the concentric rings are all different – because the delay sets up a different origin timing. (I will explain this better later – this is just a preview) Where is the beam going to go? Well it is just as obvious as the 0ms version eh?
all for now……………………….
Here are the answers to Matt’s question about what happens on the back side. It is a mirror image with the reflection going in the direction if the delay ===> Right.
Here is the same physical setup (67ms delay taper) at 63 Hz. The DELAY is the same (.67ms per box) but the PHASE shift is different – it was 30 deg per box at 125 Hz. Now it is 15 deg per box at 63 Hz. It will be 7.5 deg per box at 31.5 Hz – (not done yet). The concentric circles are rescaled to the 63 Hz size and recentered to the 15 degree increments. ( I will explain that later) Suffice to say – THESE CIRCLES ARE NOT A FEATURE OF MAPP. They are insanely (correct wording) detailed Autocad scaled drawings. There were derived by a simple 2304 step process.
This is a work in progress……….
** Update: A downloadable version of the calculator to do this work is available (courtesy of Daniel Lundberg). Go to the bottom of this post for preview and instructions.
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When a coupled array is assembled, its operating range is limited primarily by its power capability. Even very large arrays will congeal fairly quickly and once they have joined together let no phase tear them asunder. Wow! Not often that we can work hard-core religion language into speaker array theory (not to say there is not a lot of mysticism out there in line array theory land). So coupled arrays, once joined, once fully formed will maintain there shape over distance, finally either running (literally) out of air, or into the wall.
Uncoupled arrays are quite the opposite. They can’t wait to destroy themselves. The battle begins with each speaker owning its piece or real estate close by, in front of it. As we continue forward we have a happy meeting with the neighboring speaker’s response. They greet with an in-phase handshake and we have a crossover, known as the unity line. At this point the speakers are working together and the line that runs from speaker center to center (through the crossover) is approximately unity gain. This is exactly what we want to happen – an extended line of unity gain, wider than a single speaker. Ideal for frontfills, underbalconies, parade routes, racetracks and more. This is both a happy beginning AND a happy ending. How so? The beginning part is obvious, but the ending part…………well what I mean here is that this beginning is the best response we will get. It is all downhill from here as the more distant areas directly in front of each speaker no longer have sole ownership of the coverage. The others speakers are spilling in and they are arriving late. VERY late in acoustic terms. The displacement between the speakers (a factor that is large in an uncoupled array) now creates a very rapidly changing variation of time offsets between the elements. The result is combing that moves rapidly down in frequency and becomes stronger with each step we go deeper into coverage.
How far can we go before we throw up the white flag and surrender? One could evoke a variety of subjective answers such as: until it sucks, or until I can afford another set of speakers to take over etc., but these are not very satisfying to me. There is a verifiable milestone: three’s company. When we reach the point where the entire length of the coverage line is within the pattern of three sources we have reached full immersion into the combing. Three is a magic number. With three sources arrayed along a line, or an arc it is impossible to find a location that is equidistant to all three. This guarantees two or three arrival times from speakers operating within their coverage angle. That is the fight I was talking about before. The only way to stop the fight is to drown it out with another much louder speaker – like a mains to take over for your frontfill, or stop it – like a back wall for your underbalconies.
In my book I go through a set of design calculations for uncoupled line source and point source arrays. The variables are the coverage shape of the speaker (The Forward Aspect Ratio/FAR), the spacing, and the splay angle. From these we can determine where the coverage will start (D unity) and where the coverage should end (D limit). If you know the speaker and where your audience starts, you can determine the spacing, and where you will need to connect to the mains. If you have fixed positions you can get the right speaker model etc.
An example reference chart using a 80 degree speaker in an uncoupled line source is in the book. This shows nicely how to solve for this particular model and then one can refer back to the FAR chart to get the angle/FAR conversion for other speakers.
Another example reference chart uses a 90 degree speaker in an uncoupled point source source in the book. In this case the splay anglwe variable is added to the equation.
It is not possible to put an XL file into the book and not practical to give a separate chart for each speaker angle/spacing etc. but folks that bought the book don’t have a working calculator/spreadsheet that they can go to on their computer so I was in the process of making one for the blog and then Daniel Lundberg contacted me with his calculator based on this same concept. Whereas mine was derived from observing the trends and behiavior of many, many, array interactions, Daniel’s goes to the heart of the trigonometry involved.
So over the past few days I ran through some different models of speakers athdifferent angles and spacing to check for consistency between a) my published values derived through observation of other speakers at other angles
b) Daniel Lundberg’s values derived through trigonometry and geometry
c) what we can see on the MAPP plots now
The good news is they are all in very close agreement. The largest discrepancy is in the limit values for the longest range, and even these are relatively close.
Here is what the downloadable version of the calculator to do this work looks like (courtesy of Daniel Lundberg). You can have a copy of it. Free.
HOWEVER, the security rules of this blog host prohibit me from posting an XL file.
Therefore, if you want a working copy of this calculator, you will need to send me an email request to bob@bobmccarthy.com. If you think this is just a trick to get you on my mailing list…………
The second edition came off the press in October of this year and I am quite happy with the result. First thing is that the book can sit on your lap or a table without flopping around. The graphics came out much better. I was able to get them reformatted so as to get the most crtical elements like MAPP and FFT plots to the maximum size allowable on the page. I also was much more directive in getting related graphics – like a series of 4 plots, to be placed together on the page so that they can be viewed together. This was not a minor effort. Probably 80% of the graphics were reworked but this time the effort seem to pay off. When I open the book – the sizes seem right and most importantly the data is visually clear enough to prove the points being made.
I have over two years of teaching experience behind me with the old book, and this was very helpful in terms of guidance toward the second edition. I expanded on things that were found to be the most useful in my seminars and reduced some areas that were redundant as well as ones that repeated themselves or said the same thing over again, repeatedly. While the publisher gave me a goal of 25% new material and 15% longer than the 1st edition, I just felt it would be cruel and unusual punishment to add 75 pages. I did manage the 25% new material but increased the density so that the expansion is less than 8% (40 pages). I have yet to hear a complaint that my book was too short so I tried my best NOT to make it longer.
The section on cardioid subwoofers was expanded and contains a much improved explanation of how the steering occurs and how frequency affects the directionality of subs.
The discussion of the behavior of modern line array speakers was also expanded and includes the use of all-pass filters for beam steering.
The design chapters were reworked very heavily with a much more linear set of design procedures for deriving models, spacing, splay angles etc.
A final chapter was added that provides example design for a theatre, concert hall, arena and house of worship. This chapter brings together all the concepts described previously and is almost entirely graphical (about 200 new graphics were created for it).
Hopefully this will help clarify what’s new. Any comments are welcome
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